Liquid chromatography instruments often employ high performance and high pressure reciprocating pumps which use automatically operated check valves to control the liquid flow through the systems. Ball-seat check valves are often used for this purpose.
A concern associated with the use of ball-seat check valves is leakage. It has been discovered that contaminant particles in the liquid are one cause of the leakage. A partial solution to both of these problems was found in providing filters within the check valves which decrease the amount of contaminant particles in the liquid, which in turn lessens the number of contaminant particles that interferes with the ball-seat closure. Examples of ball-seat check valves of this type are found in U.S. Pat. Nos. 4,139,469 to Rainin et al. assigned to the assignee of the present invention, and 4,387,736 to Major. With particular reference to the Rainin et al. patent, the check valve described therein includes deformable washers, which cold flow to mating surfaces when subjected to a force and also serve as a filter holder. The check valve of Rainin et al. further includes a threaded housing which is engageable with a pump housing. When the housings are threaded together a compression force is generated which causes the washers and the filter holder to cold flow and seal the check valve to the pump housing. While the above-identified check valves reduce the risk of leakage between the ball and seat, leakage is still a problem. Moreover, the filters of such valves become clogged with time and must then be replaced or serviced.
From tests conducted by the assignee of the present invention, it was found in prior art valves that under high pressures certain areas of the valves often developed leaks. Three primary areas of leakage were discovered. The first area of leakage was found to occur at the ball-seat interface. It was observed that high pressures on a rigidly supported seat can cause distortions of the seat which cause leakage and even breakage of the seat. The second area of leakage was found around the ball-seat assembly, rather than through it. The joinder or abutting surfaces of the seat and the rigid support surfaces provides an alternative pathway for the fluid. The third area of leakage was found at the seals formed by the deformable washers and filter holder. The high pump pressures caused the deformable washers and filter section, of prior art check valves to cold flow radially outward over a period of time, thereby compromising the seal that was previously formed. To stop any leakage at the seals, the valve body must be tightened with a greater torque. This means that an operator has to periodically retighten the valve coupling. After several retightenings, substantial cold flow occurs in the deformable washer and a good seal is usually obtained.
It is therefore an object of the present invention to provide a check valve for use in a high performance and high pressure fluid system that is easy to assemble, convenient to install and maintain, and that has excellent sealing characteristics with little or no leakage.